1. Field of the Invention
The present invention relates to a process for producing a grain-oriented electrical steel sheet having a high magnetic flux density and used for an iron core of transformers and the like.
2. Description of the Related Art
A grain-oriented electrical steel sheet is a soft magnetic material mainly used for an iron core material of transformers and other electrical equipment and must have good magnetic characteristics including magnetic exiting and watt-loss characteristics.
The exiting characteristic is usually represented by the value B.sub.8 , i.e., a flux density obtained when a magnetic field of 800 A/m is applied, and the watt-loss characteristic is usually represented by the value W17/50, i.e., a watt-loss value per 1 kg of a magnetic material when magnetized to 1.7 T under a frequency of 50 Hz.
The magnetic characteristics of a grain-oriented electrical steel sheet are obtained through the Goss-orientation having a {110} plane parallel to the sheet surface and a &lt;001&gt; axis in the rolling direction, which is established by a secondary recrystallization during a final annealing. To obtain a good magnetic characteristic, it is important that the axis &lt;001&gt;, i.e., an axis of easy magnetization, is precisely aligned in the rolling direction. The magnetic characteristic also depends significantly on the sheet thickness, the crystal grain size, the specific resistance, the surface coating, and the steel sheet purity, etc.
The grain orientation has been greatly improved by a process characterized in that MnS and AlN are utilized as inhibitors and that the final cold rolling is carried out at a severe reduction rate. This has also led to a remarkable improvement of the watt-loss characteristic.
Recent increases in energy costs have caused the transformer makers to adopt a material having a lower watt-loss for transformers. Although materials having a low watt-loss including an amorphous alloy and a 6.5%-Si steel sheet are being developed, there are many problems to be solved in utilizing such materials in industry. On the other hand, the magnetic-domain control using a laser, for example, was recently developed, and the watt-loss characteristic has been greatly improved thereby.
The flux density is the strongest factor dominating the watt-loss, and usually the higher the flux density, the better the watt-loss characteristic. A higher flux density is occasionally accompanied by a coarsening of the secondary-recrystallized grains, and resultant degradation of the watt-loss characteristic. The magnetic-domain control, however, ensures that the higher the flux density, the better the watt-loss characteristic, regardless of the secondary-recrystallized grain diameter. For this reason, the necessity for an enhancement of the flux density has recently increased.
The production of a grain-oriented electrical steel sheet is usually carried out under extremely severe management criteria for each process step, because various factors in each step affect the magnetic characteristics. Such a way of production, however, consumes a great deal of time for management, and moreover, suffers from more than a few ill-defined degradations of the magnetic characteristics. If the magnetic characteristic of a product sheet could be predicted at an intermediate process step the above-mentioned problems of the production could be solved, but such a prediction has not yet been practically achieved despite various attempts
A currently produced grain-oriented electrical steel sheet usually utilizes MnS as an inhibitor, in which MnS is once dissolved during a slab heating for hot rolling and later allowed to precipitate during hot rolling. To dissolve MnS in an amount effective for the secondary recrystallization, a slab must be heated at a temperature of around 1400.degree. C., which is more than 200.degree. C. higher than the slab heating temperature for common steels, and has the following disadvantages.
(1) A slab heating furnace is required exclusively for the grain-oriented electrical steel sheet.
(2) The unit energy consumption of a heating furnace is high.
(3) The amount of molten scale is increased and the process operation is adversely affected; the scale must be scraped off.
Many attempts have been made to enable a heating of a slab at a lower temperature, but various problems still remain.
The present inventors and others have already disclosed a process in which a low temperature slab heating is enabled by defining the Mn content of from 0.08 to 0.45 wt % and the S content of 0.007 wt % or less (Japanese Unexamined Patent Publication (Kokai) No. 59-56522). The basic principle of this process is that the S content is reduced to ensure a [Mn] [S] product value not exceeding that obtained at 1200.degree. C. and that the secondary recrystallization is assistively stabilized by the addition of P and the heating rate of 15.degree. C./hour or slower during final annealing, etc. This process has made further progress in that the secondary recrystallization is stabilized and the magnetic characteristic is improved by the addition of Cr, as disclosed in Japanese Unexamined Patent Publication (Kokai) No. 59-190325.